FIG. 1 is a side view of part of a fastener assembly passing through a composite panel 1. The assembly comprises an externally threaded bolt 2, an internally threaded nut 3, and a washer 4. In the event of a lightning strike hitting the composite panel 1 and attaching to the fastener, sparking, plasma or out-gassing may occur at the locations 5 shown in FIG. 1.
A known method of providing spark suppression is described in EP-A-0334011. A volume of gas is enclosed by a cap around the fastener. The gas provides spark suppression for arcing that may occur between the composite structure and the metal fastener during any lightning strike.